This protocol will assess the pharmacokinetics, safety and tollerance of single and multiple oral doses or ritonavir monotherapy, and in combination with 3TC and ZDV in HIV-1 infected infants and children and to ascertain a dose or ritonavir which may be suitable for the Phase II/III evaluation of ritonavir. It will also evaluate the antiretroviral activity and the immunological effect of single and multiple doses of ritonavir adminstered alone or in combination therapy, to model plasma virus and lymphocyte kinetics within the treatment protocol, and to determine the rapidity with which virus resistant to ritonavir emerges in vivo.